For persons on a boat or travelling in remote locations, it is important to have a means of signalling one's location in emergency conditions so that rescue can be effected as promptly as possible even if such persons become incapacitated or preoccupied with survival efforts. In conventional equipment for use on water, a distress or "Mayday" call is sent by shipboard radio transmission to another ship or to a shore receiver. The recipient of the distress call typically answers and requests the sender's location coordinates. The sender must then give the coordinates over the radio, often for several times. A clear and effective transmission of the location coordinates is often not possible under conditions of extreme duress, for example, when a boat is sinking or capsized or on fire, or when the occupants are otherwise forced to abandon ship.
Conventional rescue equipment includes portable radio transmitters which are activated by persons in distress to intermittently emit a radio distress signal. However, such transmitters often do not effect a prompt rescue since the signal may not be powerful enough to reach a receiver, and they do not broadcast the location coordinates, thereby requiring the probable area of rescue to be searched in order to visually pinpoint the location of the victims.